You will be able to battle all of the Battle Tree exclusive trainers OUTSIDE of the Battle Tree during the post game.

The ROM hack doesn’t just give you more monsters to contend with at a higher level. The quality of these creatures is higher too, with the creator noting that many of the enemies come with perfect IVs—meaning, they have Pokémon with perfect stats, just like an actual player might. The creator notes that with these changes, even they actually blacked out a few times during gameplay.

Some screenshots—note the very high levels at play.

Naturally, tackling something like this would be impossible without wild monsters to help you train, so those have been edited for the hack as well.